Communication systems such as microwave systems and optical networks operating at ultra-high speeds (e.g., 100 gigabits per second (Gb/s) and greater) require a source for creating a train (sequence) of high repetition rate pulses of radiation. One approach for creating a sequence of such pulses is to perform time division multiplexing (TDM) of multiple pulse sources operating at relatively low repetition rates. In such an approach, precise timing of the relative delays for each pulse source is required.
International Patent Application No. WO 01/25849 A2, entitled “Direct Space-to-Time Pulse Shaper and Optical Pulse Train Generator,” which application is incorporated by reference herein, describes methods and systems for creating a high repetition rate pulse train using a direct space-to-time pulse generator, abbreviated hereinafter as “DSTPG.” A DSTPG is an apparatus that employs a spectral dispersing element (SDE) to spectrally disperse a single pulse of radiation to create multiple spatially separated sequences of radiation pulses. The spectral dispersing element may be, for example, a diffraction grating or an arrayed waveguide grating (AWG).
FIG. 1A is a plan view of an AWG 10. An AWG is a passive element that spectrally disperses light by virtue of an array of waveguides having different lengths and thus the ability to impart different phases to light inputted therein at one end of the waveguides. The AWG includes at least one input waveguide 12, an input slab waveguide 14 coupled to the input waveguide, an array 18 of waveguides 20 each having a different length and input ends 22 coupled to the input slab waveguide, an output slab waveguide 26 coupled to the waveguide array, and one or more output waveguides 32 coupled to the output slab waveguide. Output waveguides 32 each have an input end 36.
In AWG 10, a radiation pulse 40 enters input slab waveguide 14 from input waveguide 12 and spreads to input end 22 of waveguide array 18. The waveguide array acts as a combined lens and diffraction grating. Further, input slab waveguide 14 and ends 22 serve to spatially pattern the radiation.
Radiation pulse 40 excites pulses 42 that travel over each waveguide 20 in waveguide array 18. Pulses 42 are temporally separated by the delay increment per waveguide associated with the different length of the waveguides 20.
Radiation pulses 42 then enter second slab waveguide 26 and spectrally disperse to input ends 36 of output waveguides 32. The output waveguides act as multiple slits in that the spatial location of input ends 36 of output waveguides 32 serves to select particular wavelengths. Because of their ability to spectrally disperse and combine radiation, AWGs are commonly used in the art of optical telecommunications to multiplex and demultiplex signals.
FIG. 1B is a plan view of a modified AWG 10′, which is also described in International Patent Application No. WO 01/25849 A2. Modified AWG 10′ is similar to AWG 10, but further includes one or more waveguide operation elements (WOEs) 43 incorporated into waveguide array 18 to operate on radiation pulses 42 to form modified radiation pulses 42′. In one example, the WOEs 43 provide a fixed waveguide loss into some or all of waveguides 20. By incorporating different losses into different waveguides, the AWG can function as a DSTPG to produce an output radiation pulse sequence that is equal-amplitude (i.e., flat-topped). When the non-modified AWG 10 is used as DSTPG, it generally produces an output radiation pulse sequence that varies in amplitude.
The use of such fixed waveguide loss for generating a substantially equal amplitude output radiation pulse sequence has been described in the article by D. E. Leaird, A. M. Weiner, S. Kamei, M. Ishii, A. Sugita, and K. Okamoto, “Generation of Flat-Topped 500 GHz Pulse Bursts Using Loss Engineered Arrayed Waveguide Gratings,” IEEE Photon. Tech Lett. 14, 816–818, (2002), which article is incorporated by reference herein.
Other examples of WOEs include modifying the phase in at least one of the guides, modifying the delay in at least one of the guides, controlling or modifying the polarization in at least one of the guides, or providing amplification in at least one of the guides. In general, the WOEs are used to modify the output radiation pulse sequence produced by the DSTPG. Furthermore, WOEs 43 may be either fixed, as in a series of one or more fixed waveguide losses, or programmable, as in a series of intensity modulators.
A DSTPG can yield multiple spatially separate sequences of radiation pulses with identical temporal intensity profiles but varying center wavelength. However, having a multitude of such radiation pulse sequences on different channels is not always desired. Rather, what is often desired is a single sequence of high repetition rate radiation pulses.